1. Field of the Invention
This invention relates to ring laser gyroscope mounting devices, and more particularly to a ring laser gyroscope mount which is thermally compensated, in a very small space, to accommodate thermal changes and shock vibrations to the gyroscope frame, such that the performance of the gyroscope is relatively immune from such external forces and temperature related changes.
2. Description of Related Art
Ring laser gyroscopes are an alternative form of rotation sensors which do not require the use of a spinning mass characteristic of a mechanical gyroscope. A ring laser gyroscope employs a Sagnac effect to detect rotation optically, as an alternative to the inertial principles upon which a mechanical gyroscope operates. Planar ring laser gyroscopes, of both triangular and square geometries, have been used in inertial navigation systems and flight control systems regularly in both commercial and military aircraft. The primary advantage of the ring laser gyroscope over the spinning wheel mechanical gyroscope is its ability to withstand relatively large mechanical shock without permanent degradation of its performance. Because of this and other features the expected mean time between failures of most Ring Laser Gyroscope inertial navigation systems are several times longer than the mechanical gyroscopes they replace. The planar ring laser gyroscope was a first attempt at a non-mechanical truly strap-down inertial navigation system.
The earliest developed ring laser gyroscopes have two independent counter-rotating light beams or other electromagnetic propagation which travel within an optical ring cavity. In an ideal model of the ring laser gyroscope, these two light beams propagate in a closed loop with transit times that differ in direct proportion to the rotation rate of the loop about an axis perpendicular to the plane of the loop. However, when one steps away from the ideal model of two mode ring laser gyroscope operation, various sources of inaccuracy are observed.
The two mode ring laser gyroscope has heretofore been mechanically dithered in order to compensate for the error source effect known as lock-in. Even the most effective mechanically dithered ring laser gyroscope adds a noise component to the output of the ring laser which in turn reduces its ultimate accuracy. In an effort to achieve a fully optical ring laser gyroscope, the non-planar multi-mode ring laser gyroscope was developed to overcome the effects of mode locking without the need to dither. The term (multioscillator) refers to four modes of electromagnetic energy that propagates simultaneously in the cavity as opposed to the usual pair counter-propagating linearly polarized modes that exist in the conventional two mode gyroscope. A detailed discussion of the operation of the multi-oscillator laser gyroscope is presented in the article entitled "Multioscillator Laser Gyros" by Weng W. Chow, et. al., at pages 918-936, IEEE Journal of Quantum Electronics. Vol. QE-16, No. 9, Sep. 1980.
Although a multioscillator ring laser gyroscope provides a strap-down method of providing rotation measurement which is not subject to low rotation rate mode locking and therefore needs no dither mechanism, all ring laser gyroscopes are prone to optical pathlength changes due to thermal expansion of the gyroscope frame. Due to the multiplicity of their applications, ring laser gyroscopes are required to operate over a wide temperature range, such as -55.degree. C. to +70.degree. C. It is important that ring laser blocks used in ring laser gyroscopes neither translate nor rotate with changes in temperature, for errors can be introduced over such a wide thermal range.
When a substance is subjected to a rise in temperature, its increase in length .DELTA.l) is very nearly proportional to its initial length (l) and to the rise in temperature (.DELTA.T). If the initial and final lengths are respectively l.sub.1 and l.sub.2, and .alpha. is a proportionality constant, then, the change in length may be shown to be, EQU .DELTA.l=l.sub.2 -l.sub.1 =.alpha.l.sub.1 .DELTA.T
and the final length may be shown to be, EQU l.sub.2 =l.sub.1 +.alpha.l.sub.1 .DELTA.T=l.sub.1 (l+.alpha..DELTA.T)
where .alpha. is called the coefficient of linear expansion and depends on the substance or material that is subjected to a temperature change (.DELTA.T). The coefficient of linear expansion, .alpha., is then the change in length per unit length per degree change in Temperature (measured normally in .degree.C..sup.-1 or reciprocal Centigrade Degrees).
Typically, the ring laser gyroscope frame 82 (FIG. 3) is supported by mounts or supports fabricated of material having a different (higher) thermal coefficient of expansion than the block itself, which typically is made of low expansion material in order to maintain the integrity of length the optical pathway 55. The mounts or supports may be made from Invar, while these sleeves are mounted on Mu-Metal pedestals. The block or gyroscope frame is typically made from low expansive Zerodur.RTM. Glass. Because of the difference between the coefficients of thermal expansion of the frame and support elements, there is a tendency for the support elements to transmit thermal induced forces or torques to the frame. Such forces or torques can internally stress the block, causing misalignment of the optical components or the block, due to the change in optical pathway length. This problem occurs because of the near zero coefficient of thermal expansion of the frame and the relatively high coefficient of thermal expansion of the metal pedestal. The connection between the frame and mounting elements must be both stiff and stable, and the misalignment of optical components cannot be tolerated if gyroscope accuracy and precision is to be preserved.
The mounting system shown in U.S. Pat. No. 4,634,091 (assigned to the common assignee of this application) provides a suitable connection between the frame and mounting elements which meet the requirements of both stiffness and stability. However, the mounting design disclosed in the '091 patent may require more space than is available on some laser gyroscopes. This is the case for the multioscillator ring laser gyroscope. Only a hole of less than one inch could be allotted for the mounting hole. Such requirements for a stiff and stable mounting element, which takes up such little space, may be inadequate for the small gyroscope specification application.
FIG. 1 shows another prior configuration of a ring laser gyroscope mounting assembly. This ring laser gyroscope 10 comprises a case 14 which encloses and mounts the gyroscope frame 12 to the case 14 in a strap down configuration. Seated and securely affixed to within the central region of the frame 12 is a dither suspension 16 which includes a dither flexure 18 and a support ring 20. The support ring 20 (as described in U.S. Pat. No. 4,779,985, assigned to the common assignee of this application) has a plurality of vertically directed lengthwise slots parallel to the central axis of the support ring 20 which provide a controlled area of contact between the support ring 20 and the frame 12. The slots in the support ring 20 are arranged to permit the ring 20 to absorb thermal and mechanical stresses, preventing their transmission from the dither suspension 16 to the frame 12. A boss extends radially outward at each end of the ring 20 so that when the support ring is placed inside the cavity of the central region of the ring laser gyroscope frame, only the bosses of the ring 20 contact the frame. Flexible portions of the ring are formed by the lengthwise slots for preventing the transmission of thermal or mechanical forces between the frame 12 and dither suspension 16.
The design of FIG. 1 does not provide the flexibility for relieving stresses induced by temperature or shock, other than hoop stress, needed for the smaller area ring laser gyroscope.